1. Field of the Invention
The present invention relates to a zoom lens, which is suitable, for example, as a projection lens used for an image projection apparatus (projector) that magnifies and projects an image onto a screen.
2. Description of the Related Art
The image projection apparatus (projector) that can project an image from a personal computer or a video apparatus onto a large screen is used widely for presentation, movie appreciation and the like.
Among projection apparatuses, a so-called installation type projector is widely used, which is fixedly installed in a conference room, a movie theater or the like. In order that the fixed installation type projector adjusts to various installation conditions, it is desired that the projector be an interchangeable projection lens type projector in which multiple projection lenses can be interchanged for the same main body.
In the interchangeable projection lens projector, the position of a light bulb in the optical axis direction is determined by a standard projection lens. Therefore, for the other interchangeable projection lenses, it is necessary to adjust the axial chromatic aberration in conformity with the standard projection lens.
For the reason described above, the projection lens that is used for an interchangeable projection lens projector or the like is required to have small axial chromatic aberration. In addition, in recent years, the projection lens has been required to be a zoom lens having a high zoom ratio for the purpose of covering a large projection distance range with a smaller number of lenses. In general, when a high zoom ratio is ensured, a variation of axial chromatic aberration is increased in zooming.
In order to reduce the variation of axial chromatic aberration in zooming, there is known a zoom lens composed of lens units each configured to be achromatic by itself (US 2002/0060858).
In order to reduce the total length of the lens system while ensuring a high zoom ratio, there is known a zoom lens for a projector, which is constituted by total six lens units including lens units each constituted by a single positive lens having a large refractive power for varying magnification (US 2007/0285802, and Japanese Patent Application Laid-Open No. 2008-052174).
The zoom lens that is used for a projector is required to have a high zoom ratio and little axial chromatic aberration so that a projected image having high image quality can be obtained. In addition, the zoom lens that is used for a projector is required to have a long back focus to dispose a color separation/combination prism therein.
In order to satisfy the requirements, it is important to appropriately set refractive powers of multiple lens units constituting the zoom lens, movement amounts of the lens units for zooming, and materials of lenses constituting the lens units. If these factors are inappropriate, it becomes difficult to ensure a predetermined length of the back focus and to reduce a variation of axial chromatic aberration for zooming with a high zoom ratio so as to obtain high optical performance.